1. Field of the Invention
The present invention is directed to a circularly polarizing local antenna for a nuclear magnetic resonance apparatus of the type having a first antenna system fashioned as a frame coil and a second antenna system including a first sub-coil, with the frame coil penetrating the sub-coil.
2. Description of the Prior Art
Either linearly or circularly polarized magnetic high-frequency fields are employed for exciting the nuclei in nuclear magnetic resonance systems such as, for example, nuclear magnetic resonance imaging devices. A linear field can be conceived of as being composed of two oppositely circularly polarized fields of the same size. Only one of the two circular field components, however, is effective for the excitation. This means that twice as much power is theoretically required for an identical excitation given a linearly polarizing antenna than is required for a circularly polarizing antenna.
In medical nuclear magnetic resonance imaging devices, rules for protecting the patient only allow a maximum transmission power. This limits the possible imaging sequences, particularly the number of anatomic slices which can be scanned in a prescribed examination time. Theoretically twice as many but, in practice, not quite twice as many slices, can therefore be scanned in the same time with circularly polarizing antennas as can be scanned with a linearly polarizing antenna.
The employment of circularly polarizing reception antennas also has advantages. A circularly polarizing reception antenna is composed of two linearly polarizing antennas that receive the useful nuclear magnetic resonance signal from spatial directions that are orthogonal relative to one another. The received signals are then added in-phase after a 90.degree. phase shift of one signal. The useful signal is thus doubled, whereas the effective value of the noise is increased by a factor of square root of two. Compared to a linearly polarizing reception antenna, a circularly polarizing reception antenna thus has a theoretical signal-to-noise gain of square root of two. The signal-to-noise gain, however, does not entirely reach the theoretical value in practice. It follows therefrom that circularly polarizing antennas have advantages both in transmission as well as in reception.
German OS 31 33 432 discloses a whole body antenna or body resonator provided for generating and for receiving a circularly polarized high-frequency field in a medical nuclear magnetic resonance imaging device. Two coil groups lying opposite one another are located on an imaginary cylindrical surface, whereby the coil axes of the coil groups reside perpendicularly relative to one another.
For generating a circular, magnetic high-frequency field, the coil groups are fed with high-frequency currents that are phase-shifted by 90.degree. relative to one another. For producing tomograms, a patient to be examined is placed within the cylindrical coil arrangement in the longitudinal direction, so that the individual coils are located above and below as well as at both sides of the patient.
A local antenna for examining the head and with which circular magnetic high-frequency fields can likewise be generated, is also constructed in a way similar to the above-described whole-body antenna.
When tomograms of only one body part are registered with a conventional whole-body antenna, this whole-body antenna then also (unavoidably) receives noise signals from the other body parts because of its large reception area. The signal-to-noise ratio is thus degraded in a partial examination. Local antennas are therefore also employed for partial examinations. Such local antennas, however, are mainly constructed as linearly polarizing antennas because of their usually better manipulability and simpler signal processing. German OS 40 24 582 discloses a circularly polarizing antenna of the type initially described. The antenna is suitable for examinations at the upper body and is composed of two antenna systems. The antenna systems are each composed of a coil having a middle leg and two lateral legs, the lateral legs each having free ends connected to a return. These returns proceeding parallel to the middle legs such that there is an open winding of the two antenna systems. The circularly polarizing characteristics are present in the interior of the two coils. The characteristics required for the circularly polarizing effect, however, reside perpendicular to one another only in a limited region in the interior, so that the antenna has a non-homogeneous sensitivity or field distribution. The useful imaging volume is thus significantly smaller than the total volume of the interior. The signal-to-noise ratio of the antenna is thus also degraded in comparison to an ideal antenna having homogeneous sensitivity.